1. Field of the Invention
This invention relates to the fabrication of electrical capacitors and, more particularly, to specific high energy density capacitors and the method of fabrication thereof.
2. Description of the Related Art
Capacitors, initially and still commonly known as condensers, are one of the earliest electrical components known. They are fabricated in many different forms, shapes, sizes and types according to their particular circuit application. Their essential property, electrical capacitance, is an electrostatic phenomenon and is concerned with the storage of electrical charge and the behavior of electrons at rest. A capacitor consists essentially of two parallel plates which possess a certain capacity to store electric charge. Charges are stored on the surfaces of the two plates which are separated by a space or gap which is usually filled with some insulating material (the dielectric). The stored charges create an electrostatic field between the plates which serves to polarize the dielectric corresponding to the voltage across the plates, to which the terminals of the capacitor are formed for connection to an external circuit. The effectiveness of a given dielectric material in determining the capacitance of the component, compared with the capacitance when an air gap exists between the plates, is called the relative permittivity or dielectric constant of the material.
Barium titanate is one example of a ceramic material having a very high dielectric constant, normally in the range of 2000-12000, depending on the formation of the material, when the barium titanate is compounded with other additives. It also possesses other properties which make it an appropriate candidate as the dielectric in capacitors for storing charge at high voltages.
Because of the control which can be exercised over the chemical composition of ceramic dielectrics, there is a resulting range in possible capacitor properties. The high dielectric constants which can be achieved result in a very high capacitance per unit volume, but many of these are low voltage types. One widely used ceramic material is the rutile type, which contains varying proportions of titanium oxide and magnesium orthotitanate by which the temperature coefficient of capacitance can be closely controlled.
An extensive description of the background and methods of fabrication of ceramic capacitors, among others, is contained in an article by Donald M. Trotter, Jr. entitled "Capacitors", pp. 86ff of Scientific American, July 1988. The disclosure of that article is incorporated herein by reference.
Brown and Fischer of the U.S. Army Signal and Research Development Laboratory, in USASRDL Technical Report 2196 entitled "Properties of Hot-Pressed Barium Titanate", April, 1961, disclose that significant improvements can be achieved in the electrical properties of mechanically hot-pressed barium titanate samples prepared in the manner described in the report, as compared with conventionally prepared samples. Increases in dielectric strength from two and one-half to five times are claimed, as well as significant improvement in dielectric constant at operating temperatures below the Curie temperature. The report describes the processing of the barium titanate as involving the use of ceramic dies in a hydraulic laboratory press, the platen of which was mounted within an induction furnace. Pressures of 5000 psi were applied throughout a heating cycle having a peak temperature of 2000 degrees F.
Sintering of ceramic materials is a step which is employed to achieve highly dense ceramics. Hot isostatic pressing (HIP or hipping), a process by which an element is surrounded by a medium at elevated pressure and temperature, is commonly employed to compress certain bodies free of the application of mechanical force for the purpose of enhancing or developing desirable properties of the body. An article entitled "Gas Isostatic Hot Pressing Without Molds" by K. H. Hardtl, Ceramic Bulletin, Vol. 54, No. 2 (1975) discloses such a process used with various ceramics to develop ceramic products which are essentially pore-free. Among the uses which are mentioned are transparent materials for lamp envelopes and electro-optical applications, pore-free hard metals and ceramics for cutting tools, piezoelectric ceramics for surface wave filters, electric heating elements, magnetic recording heads and the like. In particular, the article describes a process applied to perovskites, ferrites or garnets. A normal sintering procedure is first employed to develop specimens with a closed porosity and a density in excess of 90% of theoretical. Thereafter, the specimen undergoes a HIP process to essentially eliminate the closed porosity. The article describes densifying to virtually theoretical density of various ceramics of BaTiO.sub.3, SrTiO.sub.3, Pb(Zr,Ti)O.sub.3, .sub.Al2 O.sub.3, Y.sub.3 Fe.sub.5 O.sub.12 (YIG), Mn-Zn ferrite and Ni-Zn ferrite. The article apparently is concerned with only the physical properties of the specimens undergoing the HIP process and makes no mention of any electrical properties of the materials involved.
In the HIP process described by Hardtl, the normally sintered ceramic solid is placed in the furnace (which is a water cooled autoclave) and the autoclave is closed in air. The working gas (specified as either nitrogen or argon) is pumped in up to a pressure of about 60% of the final pressure and then the furnace is heated to the desired maximum temperature in about 30 minutes. Because of the heating, the pressure inside rises to the desired maximum pressure and a control valve insures constancy of pressure. Cooling takes place over a period of about 30 minutes after hipping is concluded. The disclosure of the Hardtl article is incorporated herein by reference.
Modern apparatus for HIP consists of a high temperature furnace enclosed in a water-cooled autoclave which is capable of withstanding internal gas pressures up to about 45,000 psi and providing a uniform hot zone temperature up to about 2000 degrees C. (3632 degrees F.). The pressurization gas is commonly either argon or helium. Heating is usually effected by molybdenum or graphite resistance-heated elements argon or helium.
There are certain highly specialized needs for high energy density capacitors which are difficult to satisfy with presently available products. One such application is a mobile power source having the capability of providing electrical pulses at energy levels in the kilojoule to megajoule range. Under presently available technology, the volume required for such a power source is incompatible with the requirement for mobility. We have developed a design of product configuration and fabrication technique which reduces the volume of a 1 kilojoule unit to a minor fraction of one cubic foot. In order to deliver the stored energy in pulses of the desired waveform, rise time, etc., the power source should have very low series resistance and inductance. The capacitor power source of the present invention fulfills this need, providing a series resistance R.sub.s which is substantially less than that provided by presently known devices of a comparable nature.